This application represents a request to extend for an additional five years, an existing study of the natural history of Alzheimer's disease. The objectives of the study are to further describe the borders between normal aging and Alzheimer's disease, identify stage-specific prognoses of mild, moderate and severe dementias, examine factors which identify malignant versus benign course of AD, and provide markers for differing prognostic subgroups. Investigators will conduct follow-up studies of four cohorts each involving a combination of normal older persons, Alzheimer's disease patients, and patients with Alzheimer's disease and other causes of cognitive impairment. Collectively, these cohorts include over 1300 persons. The comprehensive evaluations include behavioral, mental status, neuropsychologic, neurologic, neuroimaging, electrophysiologic, and postmortem studies.